fatigue analyses on a jacket

8/9/2019 Fatigue Analyses on a Jacket

http://slidepdf.com/reader/full/fatigue-analyses-on-a-jacket 1/21

| Jacket Fatigue, Earthquake, Transportation Analyses in Sesam | Date: 29 October 2014 |

| Author: A.B.Berdal | www.dnvgl.com/software

JACKET FATIGUE, EARTHQUAKE AND TRANSPORTATION

ANALYSES IN SESAM

This document explains necessary steps when proceeding from static analysis of a jacket using Sesam to:

Dynamic spectral (stochastic) fatigue analysis Earthquake analysis

Transportation analysis

The 4 legged jacket shown in the figure above is used as a reference example.

To run through this example first do as explained in the document

Walkthrough example Jacket_4Leg_Comprehensive.pdf .

The following pages describe the workflow step by step with emphasis on steps demanding special

attention.





CONTENTS

1 INTRODUCTION .............................................................................................................. 3

2 STATIC ANALYSIS OF FIXED MODEL ................................................................................. 3

3 FREE VIBRATION ANALYSIS OF FIXED MODEL .................................................................... 3

4 NON-LINEAR STATIC ANALYSIS OF PILED MODEL ............................................................... 4

5 CHANGE THE MODEL USED FOR STATIC ANALYSIS ............................................................. 4

5.1 Quasi-Static Analysis 4

5.2 Dynamic Analysis 4

6 DETERMINE LINEAR SPRING STIFFNESS – SPECIAL SPLICE ANALYSIS .................................. 5

7 FREE VIBRATION ANALYSIS OF MODEL WITH SPRINGS ..................................................... 10

8 FORCED RESPONSE ANALYSIS – FIND DAFS AND DO SPECTRAL FATIGUE ANALYSIS ............ 10

8.1 Dynamic Amplification Factors (DAFs) at Various Elevations 13

8.2 Prepost Input for Extracting Forces for the Spring-To-Ground Elements and Putting

these in a G-File 14

8.3 Postresp Input for Combining Transfer Functions to Overall Base Shear and

Overturning Moment 14

9 DETERMINISTIC FATIGUE ANALYSIS ............................................................................... 16

10 EARTHQUAKE ANALYSIS ................................................................................................ 17

11 A ‘CATCH’ WHEN USING MASTER-SLAVE FOR EARTHQUAKE ANALYSIS ............................... 19

12 TRANSPORTATION ANALYSIS ......................................................................................... 20





1

INTRODUCTION

The procedure described herein is a guide in how to do some important steps in connection with the

above-mentioned types of analysis. This guide should, however, not be taken as a complete descriptionof all necessary steps for these types of analysis.

A workflow analysis process for the jacket is controlled by Sesam Manager and is illustrated below (read

columns from left to right).

2

STATIC ANALYSIS OF FIXED MODEL

A linear static analysis is performed by the activity GeniE_static_fixed_model . A model fixed at the

bottom of the legs is created, a wave load analysis in Wajac is run and a linear static analysis in Sestra isrun. Wajac and Sestra are run under the control of GeniE. The purpose of the analysis is to serve as a

basis of comparison for a subsequent non-linear structure-pile-soil interaction analysis.

3 FREE VIBRATION ANALYSIS OF FIXED MODEL

A free vibration (eigenvalue) analysis is performed by the activity GeniE_freevib_fixed_model . A

model fixed at bottom of the legs is created, added mass is computed by Wajac and a linear free

vibration analysis in Sestra is run. Wajac and Sestra are run under the control of GeniE. The purpose of

the analysis is to serve as a basis of comparison for a subsequent free vibration analysis of a model with

spring supports.





4

NON-LINEAR STATIC ANALYSIS OF PILED MODEL

A non-linear structure-pile-soil analysis is performed by the activity GeniE_static_piled_model . A

model with piles and soil is created, a wave load analysis in Wajac is run and a non-linear structure-pile-soil interaction analysis in Sestra and Splice is run:

Gensod computes soil curves/stiffnesses,

Sestra reduces the jacket, i.e. eliminates all nodes not connected to the piles,

Splice solves the pile-soil interaction non-linearly,

Sestra retracks the jacket, i.e. computes displacements and forces in nodes not connected to the

piles.

Wajac, Gensod, Sestra and Splice are run under the control of GeniE. This is an ultimate limit state (ULS)

analysis.

5

CHANGE THE MODEL USED FOR STATIC ANALYSIS

Before proceeding with the analysis some considerations must be made. These considerations are

explained in the following.

A spectral fatigue analysis in Framework may be based on a quasi-static analysis (neglecting inertia

effects in the structure) or a dynamic analysis in Sestra. In either case the wave loads are harmonic and

represented by complex loads (real and imaginary parts) hence the structural analysis as well as the

wave load analysis are in frequency domain.

5.1

Quasi-Static Analysis

In the case of quasi-static analysis the model used for static (e.g. ULS) analysis can to a large extent be

used also for spectral fatigue analysis. There are a couple of provisions though:

There should be no loads defined in GeniE or whichever preprocessor is used to create the T#.FEM

files. Such static loads are irrelevant for fatigue analysis in Framework and will also cause a failure in

Framework if included. So either delete any such loads in GeniE or create a new version of the model

without loads.

The pile-soil model must be replaced by linear spring elements at the seabed since a non-linear pile-

soil analysis in Splice is incompatible with a linear frequency domain analysis. See 6 for how to find

appropriate linear spring stiffnesses.

5.2

Dynamic Analysis

In the case of dynamic analysis the model normally needs some additional changes. Typical changes are:

In a dynamic analysis the model must be dynamically sound meaning that the stiffness must

correspond with the mass on a detailed level (each single d.o.f.) as well as overall. In a static

analysis soft parts (e.g. due to simplified modelling or low quality mesh) may be accepted since too

high static displacements in local areas may simply be neglected. In a dynamic analysis, however,

incorrect representation of mass and stiffness for a detail will influence the overall results and render

the analysis worthless.

Explicit loads (point-, line- and surface loads) representing equipment and other dead weights in a

static analysis must be replaced by mass in a dynamic analysis. In GeniE proper modelling of

equipment for static analysis is by use of either the Equipment feature or the Weight list feature

rather than explicit loads. For a dynamic analysis these equipments and weight list items should be

represented as mass rather than loads in the FE model. In the Property dialog for the loadcase(s)





containing equipments or weight lists select ‘Represent Equipment as loadcase-independent mass’.

Use either the ‘Footprint-Mass’ , ‘Beams-And-Mass’ or ‘Vertical-Beams-And-Mass’ representation. The

‘Eccentric-Mass’ option is less suitable for structural dynamic analysis and should be avoided.

There should be no loads defined in GeniE. Note that a load including only equipments and/or weight lists does not cause any problem, ref. the

item above. When the ‘Represent Equipment as loadcase-independent mass’ option has been

selected the ‘loads’ are converted to masses and the loadcase essentially ceases to exist.

The pile-soil model must be replaced by linear spring elements at the seabed since a non-linear pile-

soil analysis in Splice is incompatible with a linear frequency domain analysis. See section 6 for how

to find appropriate linear spring stiffnesses.

For a dynamic analysis a reduction technique may be required if the model is big. Of the two

reduction techniques in Sestra, the Master-Slave and the Component Mode Synthesis, the former is

the more appropriate one for fatigue analysis. If the model is split into superelements (as may be

the case if the model has been used for static analysis) then a reduction technique will necessarily be

used. Whether the model is a single or multi-superelement model the 1st level superelements must

be modified by introducing master nodes (supernodes) spread out over the model. This is required toproperly represent the dynamic energy of the model. Master (super) nodes should be defined with

the following in mind:

o Define only the three translations as super and let the three rotations be free as the

contribution of the rotations to the dynamic energy is normally modest.

o In case of a multi-superelement model remember that all six degrees of freedom of the

supernodes defined for coupling superelements together should be super to fully couple them

(only the translations as super would result in hinge coupling).

o Distribute supernodes all over the model.

o Have more supernodes where the dynamic energy is expected to be high due to large

displacements and/or due to high mass.

o Select nodes with high stiffness to be super. (The Master-Slave technique involves lumping

of mass to the master (super) nodes so low stiffness would result in too large displacementsof these nodes.)

6

DETERMINE LINEAR SPRING STIFFNESS – SPECIAL SPLICE

ANALYSIS

A procedure for determining a linear spring stiffness idealisation of the non-linear pile-soil is described in

section 14, p 14.0.1, of the document ‘Splice Engineering Documentation’ ( the document file name is

Splice_ED.pdf and is part of the Splice user documentation). The procedure described in this document is

exemplified in Case E.501, p E.07, of the document ‘Splice Verification Report’ (also found as part of the

Splice user documentation). This procedure is followed below.

A typical wave must somehow be selected for this special Splice analysis aimed at finding an equivalent

linear stiffness matrix. The wave to select may be the one contributing the most to fatigue damage or,

alternatively, the one for which 50% of the damage occurs for smaller waves and 50% for bigger waves.

One can imagine some advanced procedure for selecting this wave but the easiest may be to simply

make a qualified guess and then verify this guess based on fatigue results from Framework (using for

example the DEFINE FATIGUE-DUMP command).

For the example jacket a wave of 6 m and 8 sec. with direction from north 270º (the jacket is more

vulnerable in this direction) is chosen. (No attempt at verifying this choice is made in this example.)

First, do an analysis (GeniE including Wajac, Sestra and Splice) in which the non-linearity of the pile-soil

interaction is accounted for. This is performed by the activity GeniE_piled_model_comparison in the





sequence named Linearise_pilesoil . The load combination of interest is the selected wave combined

with gravity, equipment loading and buoyancy. Do not use any load (safety) factors in this combination

since we want a typical load level and not a conservative one. Loads from wind and current are

neglected for this typical fatigue wave case.

As result of the above analysis the file Splice.lis is found in the folder

‘…\Linearise_pilesoil\GeniE_piled_model_comparison\LinearisePileSoil’ (The subfolder LinearisePileSoil is

the analysis folder created by GeniE.) In Splice.lis find the loadcase number equal to the ‘FEM Loadcase’

number of the relevant load combination in GeniE, loadcase 10 in this example (i.e. search for the text

‘LOADCASE NO.: 10’ ), and extract the ‘PILE HEAD GLOBAL RESULTING DISPLACEMENTS’ and ‘PILE

HEAD GLOBAL RESULTING FORCES’. We are interested in the displacements and forces in the Y-direction

(since the wave has direction 270º) as well as the displacements and forces in Z-direction. For the

further calculations we select the pile with maximum compressive loading (pile 1 in this example has

compressive force 3.2775E+04). The displacements and forces extracted from the Splice.lis file are

given below. The values of interest are marked in yellow.

We will now run Splice linearly and modify the soil description in the Gensod input file so that we achieve

the same Y- and Z-displacements as for the non-linear analysis above (5.7291E-04 and 1.1555E-02).

Run this iterative linear structure-pile-soil interaction analysis as follows:

Run Gensod with the input file gensod.inp created by the activity GeniE_piled_model_comparison .

This is performed by the activity Gensod_linearise. The file SOIL.10 (containing non-linear soil

stiffnesses) is created by Gensod and Sesam Manager copies it to the repository.

Run Splice using the activity Splice_linearise but before running it edit the input file splice.inp

created by the activity GeniE_piled_model_comparison . Notice that the input file is specified to

be found on the folder ‘…\GeniE_piled_model_comparison\LinearisePileSoil\ ’. Further notice that

when editing the splice.inp file by clicking the Edit button to the right of the CmdInputFile field:

the path to the file changes:

with the effect that it is a copy of splice.inp on the workspace of the current activity that you will be

editing and not the original file. Do the following changes to the Splice input file:

o Switch off Mindlin pile-soil-pile interaction if such has been used by setting INTER=0:

PILE CON X Y Z X-DSP Y-DSP Z-DSP XX-ROT YY-ROT ZZ-ROT

1 0 -22.50 17.50 0.02 7.8050E-04 5.7291E-04 1.1555E-02 4.7256E-05 -1.6666E-04 2.2127E-05

2 0 22.50 17.50 0.02 -4.9665E-04 5.5223E-04 1.1016E-02 4.7649E-05 1.3249E-04 -2.3090E-05

3 0 22.50 -17.50 0.02 -6.4699E-04 1.6744E-03 8.1073E-03 2.7440E-04 1.2489E-04 -1.2562E-05

4 0 -22.50 -17.50 0.02 9.3199E-04 1.6555E-03 8.6387E-03 2.7505E-04 -1.5917E-04 1.3537E-05

PILE CON X Y Z X-FRC Y-FRC Z-FRC XX-MOM YY-MOM ZZ-MOM

1 0 -22.50 17.50 0.02 -2.2489E+03 2.0911E+03 3.2775E+04 1.5447E+03 1.8908E+03 2.1561E+01

2 0 22.50 17.50 0.02 2.2519E+03 2.0023E+03 3.1407E+04 1.5062E+03 -1.6000E+03 -5.0148E+01

3 0 22.50 -17.50 0.02 1.5458E+03 -9.5094E+02 2.3027E+04 -1.1363E+03 -1.4353E+03 9.4066E+00

4 0 -22.50 -17.50 0.02 -1.5488E+03 -1.0445E+03 2.4462E+04 -1.1737E+03 1.7253E+03 1.9220E+01

PILE HEAD GLOBAL RESULTING DISPLACEMENTS

PILE HEAD GLOBAL RESULTING FORCES

Non-linear solution with pile-soil-pile interaction (CONV=0.001)





o Make the analysis linear by setting the criterion for convergence higher than maximum

displacement, e.g. CONV=1 (length unit is m). You only need to do so for the loadcase of

interest, in our case loadcase 10. This section of the input looks like this (the MISC parameters

are a bit confusingly found on a second line):

Change the encircled value:

o The T1.FEM , T21.FEM and M21.SIF files created by the non-linear analysis (activity

GeniE_piled_model_comparison ) are also input to the Splice_linearise activity. These files

are copied (with their time stamp prefix as this corresponds to the Splice input) to the current

workspace using a PreExecuteScript named Copy_files_for_Splice.js. Also the file SOIL.10 found

on the repository is copied by this script.o Now run the activity Splice_linearise.

As result of the Splice run the file Splice.lis is found in the folder ‘…\Linearise_pilesoil\Splice_linearise’

In Splice.lis find the pile head global resulting displacement in Y and Z for loadcase 10 for the pile

with maximum compressive loading (pile 1) and compare with the corresponding values from the

non-linear Splice run. Compute a factor as the ratio between the displacements.

(A spreadsheet ‘Determine linear spring stiffness.xlsx ’ is found together with the input files for this

example. A screenshot of the spreadsheet is shown next page.)

See that the Y-displacement of the non-linear analysis is 5.7291E-04 while the first linear analysis is

4.7175E-04. The ratio Y-FACT (lateral resistance) is 5.7291E-04/4.7175E-04 = 1.21.

Similarly for the Z-displacement the ratio for TZZ-FCT (skin friction) and QZZ-FCT (tip resistance) is

computed as 1.1555E-02/1.0264E-02 = 1.13.

Note that second time the factors for Y-FACT, TZZ-FCT and QZZ-FCT are computed, as well as in the

subsequent iterations, the factors are computed as the previous factor adjusted by the ratio

‘displacement of the non-linear analysis’ over ‘displacement of last linear analysis’.

Modify the Gensod input belonging to the activity Gensod_linearise (see the figure below):

o Give factors for all 24 soil layers (delete all lines but one) by setting layers (LAY1 LAY2) to ‘1 24’

in the ‘DATA MODIFICATION SECTION’ for p-y, t-z and q-z.

o Give Y-FACT based on Y-displacement ratio.

o Give TZZ-FCT and QZZ-FCT based on Z-displacement ratio.





7

FREE VIBRATION ANALYSIS OF MODEL WITH SPRINGS

A free vibration analysis with the spring-to-ground stiffness found above is performed (activity

GeniE_freevib_springed_model ) to find the resonance frequencies of the jacket. Compared with themodel used for static analysis the model must be modified as explained in a previous section (Change

the model used for static analysis).

Added mass must be computed by running Wajac (with MASS command).

Run an eigenvalue analysis in Sestra using the Implicitly Restarted Lanczos (Multifront Lanczos) method.

Both Wajac and Sestra are run from within GeniE.

The first three eigenperiods found are 3.04356, 2.47933 and 1.27722. The wave periods to specify to

Wajac in the subsequent forced response analysis in frequency domain should include these eigenperiods

of the structure.

Mode shapes should be studied in Xtract (using animation if necessary) to reveal irrelevant (incorrect?)

modes. You can start Xtract from within GeniE.

8

FORCED RESPONSE ANALYSIS – FIND DAFS AND DO

SPECTRAL FATIGUE ANALYSIS

The model used for the dynamic forced response analysis (in frequency domain) is the same as the one

used for the previous free vibration analysis. A separate activity named

GeniE_fatigue_springed_model is still defined for this GeniE execution since this activity must export

the T2.FEM file. This GeniE activity is together with the other activities of the stochastic fatigue task

organised under the sequence named Stochastic_fatigue. After running the activity

GeniE_fatigue_springed_model ensure that the file T2.FEM is found in the repository.

Run Wajac for computing wave transfer functions and added mass. The execution is found in the activity

named Wajac_frequency_domain .

In the Wajac input set OPT3 = 2. on the OPTI command in order to create G1.SIF containing transfer

functions for base shear and overturning moment. Select wave periods (note that periods and not

frequencies are given as input on the FRQ commands) based on:

Eigenperiods of the structure

Cancellation and amplification effects (wave length equal half of leg distance, equal leg distance,

double of leg distance, etc.), such cancellation and amplification effects are, however, neglected in

this example

Where wave energy is high

Distribute periods more or less evenly in area of high wave energy

Ensure that the files L2.FEM , S2.FEM and G1.SIF are produced by the Wajac run and copied (by Sesam

Manager) to the repository.

Present base shear and overturning moment transfer functions by reading the G-file from Wajac (G1.SIF )

into Postresp. This is activity Postresp_glob_transf_func. Use the graph to verify (and possibly

correct) choice of wave periods. Note that you may print the transfer functions to a file and import thisinto Excel to get access to more graphing options. The Postresp commands for this are:

‘Set > Print’ and set Destination to File in the ‘Print Options’ dialog





‘Print > Response variable’ and select for instance transfer functions FORCE1 (base shear in X) and

FORCE2 (base shear in Y) in directions 0 deg. and 90 deg.

The default name of the print file from Postresp is in this case Postresp_glob_transf_func.LIS.

Now run the Sestra forced response dynamic analysis using the direct frequency response method

(activity Sestra_dynamic_frequency_domain ). Proportional damping (Rayleigh damping) has been

selected with proportional factors 0.1 for both the mass matrix and the stiffness matrix. These may be

unrealistically high and it is the user’s responsibility to give proper values for his case. Ensure that the

results file R2.SIN is found in the repository.

Now we want to compare the dynamic response with the static response. For this we can compare the

base shear or overturning moment. The static response is found in the G1.SIF file previously produced

by Wajac, see above.

To find the dynamic response we must use Prepost to extract dynamic forces in the spring-to-groundelements and put these in a G-file (G2.SIF to avoid name conflict with the existing G1.SIF ). See the

figure below (produced by Xtract) showing element numbers. The Prepost input is given in section 8.2.

Note that changing the model in GeniE (e.g. adding or deleting beams) may change the element

numbers of the spring-to-ground elements thus necessitating modification of the Prepost input. The

Prepost execution is done by the activity Prepost_extract_transfer_func. This activity has Pre- and

PostExecuteScripts that take care of the ‘non-default’ copying of R2.SIN to the Prepost workspace and

G2.SIF back to the repository. Ensure that G2.SIF is found in the repository.

In Postresp (activity Postresp_dynamic_transf_func) combine the transfer functions (the transfer

functions or response variables are termed GRES1, GRES2, etc. on the G2.SIF file produced by Prepost)

to overall base shear (add contributions from all legs) and overturning moment (add contributions from

all legs plus moment effect of vertical forces in legs). The Postresp input is given in section 8.3.

Note: To make this task simple make sure the coordinate system of the spring-to-ground elements is not

askew with the global coordinate system.

Comparison of transfer functions from Wajac and Sestra gives dynamic amplification factors (DAF) at

base level. See the graphs below comparing static and dynamic base shears in X and Y, respectively.

(The spreadsheet ‘Compare static and dynamic response.xlsx ’ producing the graphs is found together

with the input files.) Based on the graphs of the transfer functions you may want to revise the choice of

wave frequencies.

To get DAFs at various levels and not only at the bottom one must do a quasi-static analysis in Sestra

and compare this with the dynamic. This is briefly explained in section 8.1.





The spectral (stochastic) fatigue analysis in Framework may now be performed based on the dynamic

frequency domain results from Sestra. The activity Framework_stochastic_fatigue runs Framework.

Only the jacket (i.e. not the deck structure) is included in the run. A screening with high SCFs (5.0) is

first run to locate fatigue problem areas. Thereafter a new fatigue analysis is run for the fatigue problemareas using Efthymiou to compute SCFs.

Use the following command in Framework to display fatigue results:

Display > Fatigue Check Results

The figure below shows screen dumps of fatigue results with high SCFs (left) and Efthymiou SCFs (right).

8.1

Dynamic Amplification Factors (DAFs) at Various Elevations

The DAF at the base of the jacket may be found by comparing the static response as computed by Wajac

with the sum of forces computed by a structural dynamic analysis in Sestra. The base shear and

overturning moment printed by Wajac is essentially the same as the static response found from a static

analysis in Sestra. The static response is found by summing up the member forces and moments about

origin of all members connected to piles (or fixed at mudline).

The DAF at higher elevations of the jacket is found by comparing two analyses in Sestra: a static

analysis and a dynamic analysis. For a chosen elevation (horizontal section through the jacket) the

forces/moments are summed up for all intersected members. The quotient between the sum for the

dynamic analysis and the static analysis represent the DAF at that elevation. The elevation should be just above or below a horizontal bracing.

Having found the DAFs at various elevations you may perform a deterministic fatigue analysis (which is

based on a static analysis in Sestra) and account for dynamic amplification by multiplying the structural

response at the various elevations with the DAFs. This may for instance be done by multiplying the

stress concentration factors (SCFs) in the fatigue analysis with the DAFs.





8.2

Prepost Input for Extracting Forces for the Spring-To-

Ground Elements and Putting these in a G-File

% --- Create G-file containing transfer functions for spring elements

OPEN SIN-DIRECT-ACCESS ' ' R2 OLD READ-ONLY

CREATE HYDRODYNAMIC-INTERFACE ' ' G2 1

% --- First spring element

ELEMENT-FORCES 1138 1 NXX

ELEMENT-FORCES 1138 1 NXY

ELEMENT-FORCES 1138 1 NXZ

ELEMENT-FORCES 1138 1 MXX

ELEMENT-FORCES 1138 1 MXY

ELEMENT-FORCES 1138 1 MXZ

% --- Second spring element




ELEMENT-FORCES 1140 1 MXXELEMENT-FORCES 1140 1 MXY


% --- Third spring element







% --- Fourth spring element







END

EXIT

8.3

Postresp Input for Combining Transfer Functions to Overall

Base Shear and Overturning Moment

% --- Combine transfer functions for spring elements to overall transfer functions

% I.e. base shear and overturning moments (plus vertical force and twist moment)

%

% Transfer functions selected in Prepost and written to G-file are named:

% GRES1 is NXX for first spring element (as selected in Prepost)% GRES2 is NXY for first spring element

% GRES3 is NXZ for first spring element

% GRES4 is MXX for first spring element

% GRES5 is MXY for first spring element

% GRES6 is MXZ for first spring element

% GRES7 is NXX for second spring element

% GRES8 is NXY for second spring element

% etc.

%

% The correspondence between global and spring forces are:

% Global Spring

% FX -NXX

% FY NXY

% FZ -NXZ

% MX -MXX

% MY MXY

% MZ -MXZ





FILE READ SIF-FORMATTED ' ' G2

%

% --- Combine transfer functions for forces

CREATE RESPONSE-VARIABLE SHEARX 'Shear in X' GENERAL-COMBINATION ( ONLY

GRES1 -1GRES7 -1

GRES13 -1

GRES19 -1 )

CREATE RESPONSE-VARIABLE SHEARY 'Shear in Y' GENERAL-COMBINATION ( ONLY

GRES2 1

GRES8 1

GRES14 1

GRES20 1 )

CREATE RESPONSE-VARIABLE VERTICAL 'Vertical force Z' GENERAL-COMBINATION ( ONLY

GRES3 -1

GRES9 -1

GRES15 -1

GRES21 -1 )

% --- When creating transfer functions for moments one must add moments from springs

% with moment effect of forcesCREATE RESPONSE-VARIABLE MOMX 'Overturn about X' GENERAL-COMBINATION ( ONLY

GRES4 -1

GRES10 -1

GRES16 -1

GRES22 -1

GRES3 -17.5

GRES9 -17.5

GRES15 17.5

GRES21 17.5 )

CREATE RESPONSE-VARIABLE MOMY 'Overturn about Y' GENERAL-COMBINATION ( ONLY

GRES5 1

GRES11 1

GRES17 1

GRES23 1GRES3 -22.5

GRES9 22.5

GRES15 -22.5

GRES21 22.5 )

CREATE RESPONSE-VARIABLE TWIST 'Twist about Z' GENERAL-COMBINATION ( ONLY

GRES6 -1

GRES12 -1

GRES18 -1

GRES24 -1

GRES1 17.5

GRES2 -22.5

GRES7 17.5

GRES8 22.5

GRES13 -17.5

GRES14 -22.5GRES19 -17.5

GRES20 22.5 )

%

% --- Tabulate transfer functions (response variables) available on G-file

PRINT OVERVIEW RESPONSE-VARIABLE

%





9

DETERMINISTIC FATIGUE ANALYSIS

A deterministic fatigue analysis is, as opposed to a stochastic (spectral) fatigue analysis, not based on

linearity and therefore allows taking into account non-linearities such as: Load from current

Drag term of the Morison equation

Variable submergence during wave cycle

Pile-soil foundation

The non-linear pile-soil foundation is, however, often deemed less important since most of the fatigue

occurs for small to medium sized waves for which the foundation behaves mostly linearly. The pile-soil

foundation may therefore be replaced by linear spring-to-ground elements in the same way as for the

stochastic fatigue analysis.

A disadvantage of the deterministic fatigue analysis is that it involves a static structural analysis.

Dynamic effects, if important, therefore have to be approximately accounted for by use of dynamic

amplification factors (DAFs). Moreover, compared with the stochastic method the deterministic approach

requires a certain element of judgement guided by experience, especially when selecting the discrete

waves on which to base the fatigue assessment.

The model used for the deterministic fatigue analysis is basically the same as the one used for the

stochastic fatigue analysis. A separate activity named GeniE_Detfat_springed_model creates the

model in GeniE and runs Wajac (deterministic wave loads) and Sestra (static analysis) under the control

of GeniE. This GeniE activity is together with the Framework activity for deterministic fatigue organised

under a sequence named Deterministic_fatigue. To enable running Framework a PostExecuteScript for

the GeniE activity copies the R5.SIN file (produced by Sestra under the control of GeniE) to the

repository. After running the activity GeniE_Detfat_springed_model ensure that the file R5.SIN is

found in the repository.

The activity Framework_deterministic_fatigue runs the deterministic fatigue analysis. Only the

jacket (i.e. not the deck structure) is included in the run. A screening with high SCFs (5.0) is first run to

locate fatigue problem areas. Thereafter a new fatigue analysis is run for the fatigue problem areas using

Efthymiou to compute SCFs.

The figure below shows screen dumps of fatigue results with high SCFs (left) and Efthymiou SCFs (right).





10

EARTHQUAKE ANALYSIS

An earthquake analysis is based on a merged results file from two analyses in Sestra:

Static analysis Eigenvalue (free vibration) analysis

It is required that the two models are identical (same node numbering and coordinates and same

element numbering).

The spring supported model is analysed for gravity loading (including equipments) and buoyancy. The

modelling is activity GeniE_static_springed_model . To compute buoyancy Wajac is run under the

control of GeniE. (Wajac may alternatively be run after GeniE under the control of Sesam Manager of

which the free vibration analysis below is an example.) Ensure that StaticT3.FEM and StaticL3.FEM are

found in the repository after the GeniE run. The static analysis in Sestra is activity

Sestra_static_springed_model. Ensure that the results file StaticR3.SIN is found in the repository

after the Sestra run.

The results from the static analysis above will be merged with results from a free vibration analysis. The

free vibration model is created by activity GeniE_freevib_springs which produces the file

EigenvalueT3.FEM . Wajac is run by activity Wajac_added_mass to compute added mass stored in the

file EigenvalueL3.FEM . The eigenvalue analysis in Sestra is performed by activity Sestra_eigenvalue.

Sestra produces the results file EigenvalueR3.SIU .

Note that the free vibration analysis must include computation of modal earthquake excitation forces.

This is achieved by setting the MOLO parameter on the CMAS command to Sestra equal to 1. The

Implicitly Restarted Lanczos method cannot be used – use Lanczos instead. And to prepare for merging

the eigenvalue results into the static results the SIU format is chosen as format for the eigenvalue

results file (two SIN-formatted result files cannot be merged).

To distinguish the two results files prefixes are used: Static and Eigenvalue.

The merging of the results files in Prepost is done by activity Prepost_merge_earthquake. Instead of

merging the eigenvalue results directly into the static results file a copy of the static results file is made.

The original file is StaticR3.SIN and the copy is MergeR3.SIN . The copying is performed by a

PreExecuteScript named CopyResultsFileEarthquake.js for the Prepost activity. Ensure that the file

MergeR3.SIN is found in the repository after the Prepost run and that this file is bigger than the original

StaticR3.SIN . It should be bigger since the eigenvalue results file EigenvalueR3.SIU has been merged

into it.

Then run the earthquake analysis in Framework. This is activity Framework_earthquake.

To verify the total mass participation (effective modal mass) print the individual modal mass

participation factors in Framework (PRINT MODAL-MASS …). This gives the table below. Notice that the

first mode is numbered 4, this is because the merged results file taken into Framework has three static

result cases: gravity, equipment and buoyancy that are numbered 1, 2 and 3.





In the Sestra print from the eigenvalue analysis (Sestra.lis in folder ‘…\Earthquake\Sestra_eigenvalue’ )

find the mass matrix shown below. The reason for the difference in mass for the X-, Y- and Z-directions

is the directionally dependent contribution from added mass.

For the present example we find that the mass participation is 99%, 99% and 80% for the X-, Y- and Z-

directions, respectively. The Framework table over modal masses may be taken into Excel and presented

as an accumulated curve, see below. (The spreadsheet ‘Mass participation for eigenmodes.xlsx ’ is found

together with the input files.) This shows that rather few modes contribute with nearly all mass

participation. This is typical for jacket structures. The contribution to the Z-direction is, as can be

expected, from the higher modes. To achieve a high percentage contribution for the vertical direction a

high number of modes must be included.





11 A ‘CATCH’ WHEN USING MASTER-SLAVE FOR EARTHQUAKE

ANALYSIS

The ‘modal earthquake excitation forces’ (also termed ‘modal participation factors’ and ‘moda l load

factors’) are computed using formula B.353 in the Sestra user manual. In this formula j are vectors of

dimension six. The sum of squares of the elements of j can be compared to the relevant numbers in the

sum of masses reported in the Sestra listing file. The ratio (sum of squares over sum of masses) will

approach 1 from below when increasing the number of j in the sum.

If Master-Slave reduction is used then the mass for the top level superelement is the sum of the masses

of the first level superelements.

However, the j factors are computed using the reduced top level mass matrix. I.e. the Mii matrix in

formula B.353 is the reduced top level mass matrix as given in equation B.268.

When comparing this sum of squares of the elements of j to the sum of the unreduced masses the ratio

cannot be expected to approach 1 from below. It may be lower and it may be higher than 1 depending

on the chosen Master-Slave reduction.

Note that this does not mean that the Master-Slave cannot be used in connection with earthquake

analysis. Nor does it mean that it involves inaccuracies for earthquake analysis in addition to the

approximations inherent in the Master-Slave method. It only means that summing up the squares of j

cannot be used as a measure of mass participation.





12

TRANSPORTATION ANALYSIS

The model used in this case should be only the jacket without conductors and topside.

In a transportation analysis the jacket is resting on its side. It must therefore be rotated in GeniE. A way

to find the rotation angle (which will be more than 90º due to the sloping jacket panel) is as follows.

Create two auxiliary beams as shown in the figure below. The angle between them will be the rotation

angle for the jacket to make it rest on its side. (In GeniE angles between intersecting beams may be

labelled.) Deleted the two auxiliary beams are after the rotation.

The jacket is resting on its side on some supports on top of a barge. The sea-fastening is a set of

inclined beams supporting the jacket sideways and is welded to the jacket only after the jacket’s

deflection due to gravity. I.e. the sea-fastening is unloaded by the gravity of the jacket. The rolling of

the barge then subjects the jacket to rotational accelerations which put load on the sea-fastening.

Just for comparison purposes a simplified transportation analysis is performed by the activityGeniE_simple_transportation. In this analysis the effect of adding the sea-fastening after the jacket’s

deflection due to gravity is neglected. I.e. it is assumed that the sea-fastening is loaded by the deflection

due to gravity. Sestra is run under the control of GeniE.

A proper transportation analysis is then performed by the sequence named

Transportation_with_prestress.

The loading of the sea-fastening only after the jacket’s deflection due to gravity is achieved by the

following procedure:

First a static analysis with gravity load only is performed and with a very soft material assigned to

the sea-fastening. This is the two activities GeniE_transport_pre_seafast (producing the file

PreSeafastT4.FEM ) and Sestra_static_pre_seafast (producing the file PreSeafastR4.SIN ).




Then a new static analysis with rotational loads is performed, this time with normal material

properties for the sea-fastening. This is the two activities GeniE_transport_seafast (producing the

file SeafastT4.FEM ) and Sestra_static_seafast (producing the file SeafastR4.SIU ).

These two result files are then merged using Prepost. This is activity Prepost_merge_transport. APreExecuteScript named CopyResultsFileTransport.js first copies PreSeafastR4.SIN into MergeR4.SIN

before SeafastR4.SIU is merged into it.

In Xtract the gravity result case is combined with the rotational result cases. This is activity

Xtract_present_results. The figure below shows deflections and axial forces in the beams for this

combination.

fatigue analyses on a jacket

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